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Topic: 4" common anode 7-segments: driver reference to buy in 2014 (Read 27770 times) previous topic - next topic

CrossRoads

Most large digits like that are 3 or 4 or 5 LEDs in series, such as this
http://www.futurlec.com/LED/7SR40011ES.shtml
Flip all the LEDs over and it is common anode.
TPIC6c595 is perfect for driving, use 1/digit for a non-multiplexed display.
Create a font array and send out a  byte per digit:
Code: [Select]

byte fontArray[] = {
0b00111111, // 0
0b00000110, //  1
:
:
0b01101111, // 9
};

0bDP-g-f-e-d-c-b-a, a bit =1 is an on segment
    a
f          b
     g
e          c
     d          DP
Code: [Select]

digitalWrite (RCLK, LOW);
SPI.transfer (fontArray[numberToDisplay[0]]); // double lookup!
SPI.transfer (fontArray[numberToDisplay[1]]);   // data goes on MOSI with SCK to SRCLK pin
SPI.transfer (fontArray[numberToDisplay[2]]);
SPI.transfer (fontArray[numberToDisplay[3]]);  // OE/ tied Low or PWM for brightness, SRCLR tied to +5
digitalWrite (RCLK, LOW); // data outputs change on this rising edge

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

PaulRB



An array in a chip with only the logic level power FETs would be ideal for other applications.


That would be a useful part to keep in our part boxes. Do we know of one?


This is the only thing I could find:

http://www.digchip.com/datasheets/download_datasheet.php?id=2605607&part-number=AN01&show=inline

CrossRoads

Continuous current rating is very low. TPIC6x595  much better in place of this, 8 outputs with 3 control pins vs needing 8 on this device.
Check Mouser for current availability of Supertex parts.
http://www.mouser.com/Search/Refine.aspx?Keyword=supertex
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

PaulRB


Continuous current rating is very low. TPIC6x595  much better in place of this, 8 outputs with 3 control pins vs needing 8 on this device.
Check Mouser for current availability of Supertex parts.
http://www.mouser.com/Search/Refine.aspx?Keyword=supertex


Agreed, it looks inferior to tpic6c595. But Paul__B posed the question is there a fet equivalent to uln2803, which has a rather high voltage drop (~1V) due to its use of darlingtons.

PaulRB

#19
Sep 03, 2014, 10:05 am Last Edit: Sep 03, 2014, 10:21 am by PaulRB Reason: 1
Mat,

Looks like uln2803 will be the most cost effective part part. However, to be sure it will be suitable, you need to measure the forward voltage of those segments, to confirm it is around 10.5V. Connect a 1K resistor  to one segment and your 12V to the anode. This should not light at all. If it does light anthing more than a faint glow, we have the wrong data sheet!

Assuming 1K does not work then try 100R. Hopefully the segment will light, although it may be dim. If it does not light, try a lower value like 68R. When the segment lights, measure the voltage accross the segment and accross the power supply (which is regulated or switch-mode, yes?).

Once you have measured the forward voltage, you can then calculate the correct resistor to give a nice bright display.

PS. Uln2003 is similar to uln2803 but with only 7 outputs, so that will also be suitable, if you do not want to use the dp.


However, if in the future you wish to expand the project and connect more displays or other things to the arduino then the tpic6c595 will leave more Arduino pins free.

Thanks @CrossRoads for the code sample !
and @PaulRB and Paul__B for the explanations



I did the measurements:
with 12V supply and 330ohm, the voltage is 7,60V
the light is really super bright! too much! under my screens, it blinds me  8)

with 12V supply and 10k, it's dim and voltage is 6,9V

with 8,2V (from a transformator), 2kohm: voltage is 6,9V
it fine as well: I suppose I'll do that!

What do you think about this?  :smiley-roll:


CrossRoads

What are you using for a driver, and where is 6.9/7.6V being measured?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

sorry!
I don't use any driver: the segment and the resistor are connected in serial to the PSU


I tried to follow the instructions of PaulRB:

r= 2kohm
Vinput = 8,2V
Vf = 6,9V

PaulRB

#23
Sep 04, 2014, 11:29 pm Last Edit: Sep 04, 2014, 11:48 pm by PaulRB Reason: 1
Hi Mat.

The voltage you are giving for the supply: did you measure that yourself? Are the 8.2V and 12V supplies unregulated, regulated or switch mode?

What I think your measurements mean is that we have not identified the correct data sheet for your displays. We should have looked for something with a forward voltage of 6.9 to 7.6 V.

With 12V and 330R you measured 7.6V accross the segment. This means the voltage accross the resistor is 4.6V and the current flowing is 13.3mA. With 8.2V supply and 6.9V accross the segment and a 2K series resistor, less than 1mA will be flowing.

it seems strange to me that the voltage accross the segment varies by 0.7V

If the display is too bright with 330 R, try 680R or 1K.

Whatever, a uln2803 is going to be ok, but will use up 7 or 8 Arduino outputs. So before you make a final decision, answer my earlier question: do you wish to expand each Arduino to drive other displays or other circuits? If not, then using 8 outputs for the Uln is not a problem for you.

PaulRB

This may be a better match for your displays:

http://www.led-display.cc/Single-Digit-LED-Display/67/LD40011CD-4-Inch-Single-Digit-7-Segment-LED-Display.htm

The "ultra red" for example has a typical forward voltage of 7.6V. Each segment contains 8 leds, 4 pairs in parrallel. This may explain why they are so bright!

#25
Sep 05, 2014, 10:11 am Last Edit: Sep 05, 2014, 10:13 am by Mat13 Reason: 1
Thanks PaulRB

>Are the 8.2V and 12V supplies unregulated, regulated or switch mode?
it is not professionnal device! I'm not sure I can answer to this question...  :smiley-roll:
It's this device :


The link seems broken...

I may want to more functions to my dashboard, so I will use a driver like 6c595

If I want to adjust the brightness, can I connect in serial a single potentiometer?

PaulRB

#26
Sep 05, 2014, 10:39 am Last Edit: Sep 05, 2014, 11:07 am by PaulRB Reason: 1
I can't really say much about your power supply from that photo, it is too blurred.

Dimming leds with potentiometer: no, leds don't really work like that. To dim them it is better to use the PWM method. Pulse Width Modulation is where the leds are switched on and off at high speed (too fast for the eye to see). The brightness is controlled by the % of time the leds are on.

This is easy to do with Arduino and tpic6c595. The arduino has PWM outputs and the '595 has a pin called Output Enable (OE) to connect it to.

By connecting a pot to an arduino analog input, you can then adjust the brightness. Or better still, connect a light dependant resistor (LDR) and the arduino can then adjust the brightness automatically to suit the conditions.

Ok,
I understand that dimming with a resistor (versus PWM) is not efficient (higher power consumption?)

Good point if 6c595 supports PWM: thanks for this good news
and on the other hand, I know PWM with Arduino  :)
(with coding_badly we achieved ultrasonic PWM here http://forum.arduino.cc/index.php?topic=135847.msg1706807#msg1706807 )

1- The power supply is probably not a switch-mode power supply: old and heavy!
is it a major issue?

2- Should I rather use 8V or 12V power supply? What is the most interesting to do?

PaulRB

To both questions: depends on that power supply. Is it regulated? Does it measure 12V with no load on 12V setting, and same question on 8.2V?

If not, you will need to add a regulator.

If regulated, it will be the same efficiency to use either voltage. Either the regulator in the power supply the series resistors in your curcuit will dissipate the unwanted power.

>If not, you will need to add a regulator.
I have found these step-down regulators that I would fed with 12V to output 8V regulated
http://www.dx.com/p/dc-dc-27v-to-24v-12v-3v-buck-car-power-constant-current-regulator-circuit-board-285001#.VAqrf2Pisxk
or this:
http://www.dx.com/p/mini-dc-dc-adjustable-voltage-regulator-module-blue-151211#.VAqrlWPisxk

Do you have an advice?
Thks

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